The present invention relates to a cylinder head for an internal combustion engine, and more particularly relates to such a cylinder head for an internal combustion engine for a vehicle such as an automobile, particularly which has a valve seat maunfactured integrally therein and endowed with exceptional wear resistance.
The present invention has been described in Japanese Patent Application Ser. No. Showa 60-296191 (1985), filed by an applicant the same as the applicant or the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the text of said Japanese Patent Application and the claims and the drawings thereof; a copy is appended to the present application.
In the prior art, internal combustion engine cylinder heads have been often formed of aluminum alloys of various types, because of the advantages conferred upon such a material in view of its low weight and easy formability. However, the valve seat portions of such a cylinder head are very liable to wear, because they are repeatedly impacted by the intake or the exhaust poppet valves while the internal combustion engine is in operation, while simultaneously being exposed to extreme conditions of temperature and shock and gas abrasion and the like caused by the explosion of air-fuel mixture in the combustion chambers of the engine. As a result of this, in order to prevent excessive wear of and/or braking up and away of the valve seat portions of such a cylinder head made of aluminum alloy, due to friction and heat damage from frictional contact with the intake and the exhaust valves and with the products of combustion such as flaming gases, and also in order to limit dimensional changes of the valve seat portions due to thermal expansion caused by said hot combustion products, it has been in the past practiced to form a recess in the valve seat portion of the cylinder head by valve seat grinding or the like, and to insert, for example by cold pressing, into this recess a ring shaped valve seat insert made of cast iron or iron based sintered material.
However, in such a conventional type of cylinder head with pressed in insert valve seat, there is the problem that it is quite common for a thermal air insulation layer to be formed between the pressed in valve seat and the material of the cylinder head, and as a result of this, when under the operating conditions of the internal combustion engine the cylinder head and the valve seat are exposed to extremely high temperature, the transmission of heat from the valve seat to the cylinder head can be substantially obstructed by this thermal air insulation layer. As a result of this, the valve seat is liable to become unduly hot, particularly at particular portions thereof corresponding to points at which said thermal air insulation layer is thicker than at other points thereof--for the thermal air insulation layer is typically not of the same thickness all around the valve seat. In such a case, deterioration of various operational characteristics, including deterioration of the wear resistance of the valve seat, are liable to occur.
Furthermore, since the coefficent of thermal expansion and the coefficient of thermal conduction of such cast iron or sintered iron based material of which the valve seat is typically made are very different from the corresponding coefficients relating to the aluminum alloy of which the main body of the cylinder head is made, thereby, when the valve seat is pressed into the cylinder head itself, in view of these differences in thermal conductivity and thermal expansion, very high dimensional accuracy of the valve seat and of the recess in the cylinder head for receiving it become crucially important, and thus complicated and painstaking machining processes come to be required. This inevitably entails high cost.
A further problem that is encountered is that it is necessary to determine the strength and the dimensions of the portion of the cylinder head itself which receives the valve seat to be sufficient to reliably hold the valve seat when said valve seat is pressed into said cylinder head portion, and therefore the diameter of the valve seat and the diameters of the intake and the exhaust poppet valves come to be restricted, and it is in such a case difficult to increase the cooling efficiency of the cooling system for the cylinder head by making the coolant passages within said cylinder head very closely approach the valve seats and the combustion chambers. Thus, it becomes difficult to provide high performance of the internal combustion engine.
In order to resolve the above outlined problems with regard to forming the valve seat portions in a conventional cylinder head, methods of alloying which may be considered applicable to the formation of a valve seat portion of a cylinder head have been described in Japanese Patent Laying Open Publications Ser. Nos. 55-8497 (1980) and 57-171572 (1982), neither of which is it intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law. In these methods, there is disclosed the concept whereby the surface portion of a metal sample made of a base metal material is melted by the use of a high energy source, an alloying material is added to this melted base metal surface portion, and then the melted and alloy portion is rapidly cooled down by absorption of heat by the other portions of the sample, whereby an alloy layer is formed on the surface of the base metal portion, comprising the base metal material and the alloy material alloyed therewith. Furthermore, in SAE Technical Paper Series 850406 there is described an internal combustion engine cylinder head in which a valve seat surface is defined by using such a local surface alloying method.
When the above type of alloying method is applied to the formation of a valve seat portion of a cylinder head which is made of aluminum alloy, all of the alloying material is melted into the aluminum alloy base material, and therefore a layer of substantially only the alloying material is definitely not formed on the surface of the alloy layer which is formed. Thus, the surface layer of the valve seat is in fact an aluminum alloy of a different composition from the aluminum alloy which makes up the main body of the cylinder head, but which is manufactured therefrom by alloying thereto the added alloying material. As a result of this, it is difficult satisfactorily to improve the wear resistance characteristics and so on of the valve seat. Again, when elements such as silicon and nickel and so on are added as alloying materials in order to improve the wear resistance and the heat resistance and so on of the valve seat, primary crystalline silicon and metallic compounds such as nickel-aluminum are formed, and these come to be distributed finely throughout the alloy layer. Such primary crystalline silicon and metallic compounds such as nickel-aluminum have good heat resistance, but, since the basic composition of the surface of the valve seat is inevitably that of an aluminum alloy material, if such a valve seat is exposed to a relatively high temperature such as about 150.degree. C. for at least about 100 hours continuously, then the strength, the heat resistance, and the like of the valve seat surface will inevitably be severely deteriorated and in the worst case the valve seat will completely fail. Thus, such an aluminum alloy layer type valve seat surface is not durable enough for practical use in an internal combustion engine. If, in order to improve the heat resistance, the amount of the above described primary crystalline silicon and metallic compound such as nickel-aluminum is increased, then although such distributed materials have good heat resistance, since their toughness is extremely small and is in fact close to zero, a hard but extremely brittle alloy layer is formed as the valve seat surface. In an alloy layer thus formed, the problem arises that cracks may have already occurred after the formation thereof and even before the use thereof, and therefore such an alloy layer, although it can be described and conceived of from a theoretical standpoint, is not a practically useful material.
Also, with an alloying type surface preparation method such as described above for a valve seat surface for a cylinder head of an internal combustion engine, the rate of cooling of the alloy layer decreases in order from the interface with the main body portion of the cylinder head, the interior of the alloy layer, and the surface of the alloy layer, and it is not possible to ensure a uniform rate of cooling for all portions of the alloy layer, as a result of which it is difficult to obtain a uniform composition of the alloy layer, and in particular it is difficult to make the wear resistance of the surface of the alloy layer high, and therefore in the formation of the valve seat portion of the cylinder head a thick alloy layer is formed, and it is necessary in practice to apply a machining process with a relatively high process cost to the surface of the alloy layer.